Power Recovery Process

ABSTRACT

Processes using multiple expansion turbines for efficient recovery of power from a plurality of very high pressure streams of superheated vapor are disclosed. Beneficially, processes of the invention use at least two classes of expansion turbines. Processes according to this invention are particularly useful for recovery of power from very high pressure streams of superheated steam in an olefins manufacturing process. Such streams are typically produced by thermal cracking of suitable petroleum derived feed stocks, and the olefins being produced and purified are typically ethylene and/or propylene.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under United States Department of Energy Cooperative Agreement No. DE-FC07-O1ID 14090.

FIELD OF THE INVENTION

The field of this invention relates to use of multiple expansion turbines for efficient recovery of power from a plurality of very high pressure streams of superheated vapor. More particularly, these power recovery processes employ at least two classes of expansion turbines. Processes according to this invention are particularly useful for recovery of power from a plurality of very high pressure streams of superheated vapor generated in manufacturing petrochemicals. High pressure streams of superheated steam are typically produced during thermal cracking or pyrolysis of suitable petroleum derived feed stocks. For example, superheated steam is generated where olefins, typically ethylene and/or propylene, are produced.

BACKGROUND OF THE INVENTION

As is well known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Olefins including, importantly, ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.

Commercial production of olefins is almost exclusively accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feed stocks include streams of ethane, propane or hydrocarbon liquids ranging in boiling point from light straight-run gasoline through gas oil.

This endothermic process is carried out in a plurality of large pyrolysis furnaces with the expenditure of large quantities of heat which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is cooled and put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirements for the process are thus very large and ways to reduce the net energy use of olefins manufacturing facilities are of substantial commercial interest.

Each of the plurality of pyrolysis furnaces produces a byproduct stream of very high pressure superheated steam. These streams are typically combined and directed to one or more multi-stage expansion turbines which produce power for use in the cryogenic separation system. The efficient conversion of the multiple very high pressure superheated steam streams into mechanical energy is crucial for the economic production of olefins. Processes which allow more efficient conversion of very high pressure steam into mechanical energy or electricity, such as the process of the present invention, beneficially reduce the net energy use of the olefins manufacturing facility.

It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods for recovery of power from a plurality of very high pressure vapor streams, and generate turbine exhaust streams at a plurality of pressures

An improved method for recovery expansion power should exhibit higher efficiency thereby providing lower net energy use and therefore lower variable costs of operation.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.

SUMMARY OF THE INVENTION

Economical processes are disclosed for the use of multiple expansion turbines for efficient recovery of power from a plurality of very high pressure streams of superheated vapor. More particularly processes are disclosed for recovery of power using at least two classes of expansion turbines. Processes according to this invention are particularly useful for recovery of power from very high pressure streams of superheated steam generated in the manufacture of light olefins by the pyrolysis of hydrocarbons in a plurality of furnaces. Heat is removed from the furnaces and/or reactor effluent streams at least in part by the formation and removal therefrom of a plurality of very high pressure steam streams.

Processes of the invention comprising a power generation system employing a plurality of steam expansion turbines wherein high-pressure steam is expanded to produce power and generate turbine exhaust streams at a plurality of pressures. More particularly, this invention comprises power recovery processes employing at least two classes of expansion turbines, which comprise: (a) expanding a first stream of superheated vapor at first inlet conditions, including temperature and pressure, to obtain at least one first expanded stream of superheated vapor at first intermediate conditions using at least one primary class expansion turbine to thereby recover a first amount of power; (b) combining two or more vapor streams into a single very high-pressure superheated vapor stream; (c) cooling the resulting single very high-pressure stream from step (b) by indirect heat exchange with at least a portion of the first expanded stream from step (a) to provide all or a portion of the first stream of superheated vapor for expansion in step (a), and a resulting heated first expanded stream at second intermediate conditions including a second intermediate temperature; (d) expanding at least a portion of the resulting heated stream from step (c) at second expansion inlet conditions to obtain at least one second expanded stream of superheated vapor at third intermediate conditions using at least one secondary class expansion turbine to thereby recover a second amount of power.

In a particularly useful aspect of the present invention, three or more vapor streams are combined in step (b) into a single very high-pressure superheated vapor stream. Advantageously, three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are derived from a petrochemical process.

In another aspect of the present invention, the vapor comprises a light organic compound component containing from about 2 to about 4 carbon atoms, for example propane. The temperature differential between the second intermediate temperature and the first inlet temperature is advantageously no more than 100 Fahrenheit degrees. More advantageously, the temperature differential between the second intermediate temperature and the first inlet temperature is no more than 70 Fahrenheit degrees.

In another particularly useful aspect, this invention comprises power recovery processes employing at least two classes of expansion turbines, which comprise: (a) expanding a first stream of superheated steam at first inlet conditions of temperature and pressure to obtain at least one first expanded stream of superheated steam at first intermediate conditions using at least one primary class expansion turbine to thereby recover a first amount of power; (b) combining three or more streams of very high pressure steam into a single very high-pressure superheated stream; (c) cooling the resulting single very high-pressure stream from step (b) by indirect heat exchange with at least a portion of the first expanded stream from step (a) to provide all or a portion of the first stream of superheated vapor for expansion in step (a), and a resulting heated first expanded stream at second intermediate conditions including a second intermediate temperature; (d) expanding at least a portion of the resulting heated stream from step (c) at second expansion inlet conditions to obtain at least one second expanded stream of superheated steam at third intermediate conditions using at least one secondary class expansion turbine to thereby recover a second amount of power.

A particularly useful aspect of the present invention further comprises treating at least a portion of one or more second expanded stream of superheated steam from step (d) to thereby provide at least a portion of the resulting single very high-pressure stream of step (b). Beneficially, three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are generated in a process for thermal cracking of suitable petroleum derived feed stocks to produce olefins, and advantageously the olefins being produced are ethylene and/or propylene.

In another particularly useful aspect of this invention three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are generated in a process for the manufacture of light olefins by the pyrolysis of hydrocarbons in a plurality of furnaces from which heat is removed in a plurality of very high pressure streams of superheated steam. The first inlet pressure is at least 900 psig, and/or the first inlet temperature is at least 800° F. In yet another useful aspect of this invention, the second intermediate temperature is no more than 100 Fahrenheit degrees below the first inlet conditions temperature.

Yet another particularly useful aspect of the invention further comprises partially desuperheating the second expanded stream of superheated steam from step (d) by indirect heat exchange with a cooling medium to provide a supply of superheated low-pressure steam. More advantageously, the cooling medium is boiler feed water, and at least a portion of the heated boiler feed water is used to produce at least a portion of the very high pressure steam streams of step (b).

Efficiency is improved in accordance with this invention for any power recovery steam cycle in which there are a relatively large number of steam generation and superheating units and where there are multiple pressure levels from which steam is expanded to produce power. The reheating of an intermediate-pressure steam, which has been extracted from an expansion turbine, in a single location by desuperheating a higher-pressure steam that has been superheated in the multiple steam superheating units is critical for best results. This invention avoids the complex and expensive system that would be needed to distribute and then re-collect the intermediate pressure steam if it were reheated in the multiple superheating units.

For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawing and described below by way of examples of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The appended claims set forth those novel features which characterize the present invention. The present invention itself, as well as advantages thereof, may best be understood, however, by reference to the following brief description of preferred embodiments taken in conjunction with the annexed drawings, in which:

FIG. 1 is a schematic diagram of a comparative power recovery process for the steam system in an olefins manufacturing thermal cracking unit.

FIG. 2 is a schematic diagram of an embodiment of this invention in which intermediate-pressure steam is heated and low-pressure steam is desuperheated.

FIG. 3 is a schematic diagram of another embodiment of this invention in which reheated intermediate-pressure steam is reheated and distributed to multiple expansion turbines.

It should be noted that only essential expansions and heating/cooling steps are shown in these schematic diagrams.

BRIEF DESCRIPTION OF THE INVENTION

Hydrocarbon cracking processes have been commonly employed in the petroleum and allied industries for several decades, and many commercial cracking processes have been the subject of much academic and commercial interest. Cracking consists of breaking down the hydrocarbon molecules into smaller molecules, usually at a higher temperature. There are generally two types of cracking, thermal cracking and catalytic cracking, which utilize either the effect of temperature alone or in combination with the active sites of a catalyst.

In a conventional thermal cracking unit, the hydrocarbon feedstock is gradually heated in a tubular furnace. The thermal cracking reaction takes place mainly in the portion of the tubes receiving the maximum heat flow, and the desired temperature is determined by the nature of the hydrocarbons to be cracked.

In general, a cracking unit includes a plurality of pyrolysis furnaces. Each furnace includes a tubular or plug-flow reactor through which feedstock flows and in which the feedstock is thermally decomposed. A pyrolysis furnace is designed to transfer heat to internal reactor tubes which are conventionally arranged in three sections: a convection section, in which the hydrocarbon feedstock is preheated and very high pressure steam is superheated; a radiant section, in which the preheated hydrocarbon feedstock is thermally decomposed to olefins, diolefins, and aromatics; and a quench section where the cracked gas furnace effluent from the radiant section is cooled through the generation of very high pressure steam.

The literature is replete with disclosures of suitable pyrolytic furnaces for the thermal cracking of hydrocarbons. For example, U.S. Pat. No. 5,271,809 in the name of Hans-Joachim Holzhausen. Pyrolytic furnaces advantageously comprise a radiation zone including burners and cracking tubes in the radiation zone consisting of parallel, vertically extending linear tube sections joined to one another by tube elbows located in the bottom region of the radiation zone. At least four cracking tubes are combined into groups uniformly arranged in the radiation zone, each group of cracking tubes being united in an outlet tube via manifold tube sections wherein the linear tube sections and the manifold tube sections of the individual groups are arranged in one row in the transverse direction of the pyrolytic furnace.

When light olefins and monoaromatic compounds are to be produced, the necessary temperature is and generally ranges from about 1,440° F. to about 1,600° F., depending on the type of feedstock to be cracked, but is limited by the operating conditions of the process and by the operating complexity of the furnaces, which use supplementary heating energy.

Suitable light hydrocarbon fraction or fractions may be advantageously chosen from the group consisting of light paraffins, such as ethane, propane and the butanes, and heavier hydrocarbons such as gasolines, naphthas and gas oils, and even certain higher-boiling but strongly paraffinic or naphthenic fractions, such as the paraffins or slack wax or the hydrocarbon recycles. These hydrocarbon fractions may come from different units of the refinery, for example the atmospheric distillation, visbreaking, hydrocracking, oil manufacturing or olefin oligomerization units, or from the effluents of the conversion unit itself. Additionally, the various fractions may be injected either alone or in combination with steam and optionally other gases such as hydrogen or light gases.

Each olefins-producing pyrolysis furnace typically produces one or more streams of high-pressure superheated steam as a byproduct of the furnace operation. The steam is typically generated through the quenching of hot furnace effluent gases, and then superheated in the convective section of the pyrolysis furnace. The maximum temperature to which the steam is superheated is typically limited by the maximum inlet temperature of the expansion turbine to which the superheated steam is fed. The maximum inlet temperature of the expansion turbine is in turn a function of the design and metallurgy employed in the turbine. The maximum inlet temperature of steam turbines is typically in the range of 980 to 1000° F.

The superheated steam streams are beneficially combined and enter one or more multi-stage expansion turbines. These turbines produce mechanical and/or electrical power which is beneficially used in the recovery and purification of olefins.

The recovery and purification of light olefins such as ethylene and propylene from the furnace effluent is an energy-intensive process. A typical ethylene recovery and purification section comprises a cracked gas compressor to compress the quenched furnace effluent stream to a relatively high pressure, typically between 200-500 psig. At least a portion of the mechanical energy required for cracked gas compression is produced through the expansion of the very-high pressure steam generated in the pyrolysis furnaces.

The ethylene contained in the compressed cracked gas is then typically recovered and purified through cryogenic distillation. While the design of the ethylene recovery and purification section admits of many variations, it typically contains a deethanizer tower to separate C3 and heavier material from the ethylene-containing stream, a demethanizer tower to separate methane and lighter material from the ethylene-containing stream, and a C2 splitter tower to separate ethylene from ethane. Such distillation steps are typically cryogenic in nature, that is they are carried out at temperatures below ambient temperature. They therefore demand significant amounts of process refrigeration. At least a portion of the mechanical energy required for providing the process refrigeration is produced through the expansion of steam generated in the pyrolysis furnaces.

In the course of its extensive work in this field, the applicants have found that use of multiple expansion turbines increases the efficiency in the recovery of power from a plurality of very high pressure streams of superheated vapor generated and superheated among a relatively large number of furnaces. More particularly, power recovery processes in accordance with the invention employ at least two classes of expansion turbines. Reheating of intermediate-pressure exhaust or extraction vapor from the primary class of expansion turbines is critical for improving the efficiency of the overall power recovery system.

This invention is useful for power generation systems from a plurality of streams at very high pressure that use any working fluid, though steam is by far the most common. Processes of this invention are particularly suitable for use in the thermal cracking of hydrocarbons, for example, using steam streams from a plurality of thermal cracking furnaces.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

While this invention is susceptible of embodiment in many different forms, this specification and accompanying drawings disclose only some specific forms as an example of the use of the invention. In particular, a preferred embodiment of the invention for recovery of mechanical and/or electrical power from a plurality of high pressure superheated vapor streams is illustrated and described. The invention is not intended to be limited to the embodiment so described, and the scope of the invention will be pointed out in the appended claims.

The apparatus of this invention is used with certain conventional components the details of which, although not fully illustrated or described, will be apparent to those having skill in the art and an understanding of the necessary function of such components. Various values of compositions, flow rates, temperatures, and pressures are given in association with a specific example described below; those conditions are approximate and merely illustrative, and are not meant to limit the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention represents an improved, more energy-efficient method for utilizing high-pressure superheated vapor generated from multiple sources to generate mechanical energy through the use of expansion turbines. It can be utilized with any high-pressure superheated vapor, but a common use would be in a steam system where the vapor is water vapor. For ease of understanding the invention will be described in terms of an improved steam system for the generation of power within an olefins manufacturing complex. It should be noted that the concept and methods of this invention are not limited to this application.

In olefins manufacture, high-pressure steam is generated in a number of cracking furnaces. The number of cracking furnaces in a particular olefins unit will depend on many factors, including the capacity of the olefins manufacturing unit, the capacity of the furnaces, and the design of the furnaces. Typically between four and 12 furnaces are utilized within an olefins manufacturing complex. Each of these furnaces produces a stream of superheated steam as a byproduct of the olefin-producing process. These streams are typically combined and then directed to steam turbines to produce mechanical energy. The mechanical energy thus produced is typically used to compress the olefin-containing gas and to drive machinery designed to provide refrigeration to the olefins process.

FIG. 1 depicts a schematic diagram of a portion of a conventional steam system for an olefins manufacturing unit. This schematic contains only the major heat transfer and power production steps that are required to understand the basic operation of such a steam system, and to allow comparison with the current invention. Those skilled in the art will recognize that olefins unit steam systems admit to many variations in design, but most contain the steps outlined in FIG. 1.

Very high pressure superheated steam is generated by the multiple furnaces and combined into a single very high-pressure steam header line depicted as stream 1. The temperature and pressure of stream 1 can vary significantly between units. Stream 1 is typically at a pressure of at least 900 psig and a temperature of at least 900° F.

The entirety of this very high pressure steam is typically directed to steam turbine 2. This steam turbine expands the very high-pressure steam to produce power for other parts of the process. Typically, the power derived from steam turbine 2 would be used to drive a cracked gas compressor to compress the cooled olefin-containing furnace effluent gas to a higher pressure. Turbine 2 is shown as an extracting turbine, with two stages (stage 2 a and stage 2 b) which are typically mechanically coupled. High-pressure steam (typically at about 600 psig) is recovered from stage 2 a as stream 3.

A portion of stream 3 is directed as stream 4 to stage 2 b of the turbine and withdrawn as stream 5. Stream 5 is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium.

Another portion of stream 3 is directed as stream 6 to the high-pressure steam header. Portions of the high-pressure steam from the header, depicted as streams 7 and 8, can be directed to other steam turbines, depicted as 9 and 10. It is understood that more or fewer turbines can be fed by the high-pressure steam header, depending on the needs of the olefins process. In order to simplify the FIG. 1, only two turbines are depicted.

A further portion of the high pressure steam can be directed as stream 11 to one or more heat exchangers to provide heating to one or more units in the olefins process. While a single exchanger 12 is shown in FIG. 1, it is understood that it may represent multiple heat exchangers in a commercial olefins facility. The condensate stream 13 from exchanger 12 is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A final portion of the high-pressure steam can be exported as stream 14 to another process or otherwise used within the olefins unit.

In FIG. 1 steam turbine 10 is shown as an extracting turbine, with two stages (stage 10 a and stage 10 b) which are typically mechanically coupled. Low-pressure steam (typically at about 65 psig) is recovered from stage 10 a as stream 15.

A portion of stream 15 directed as stream 16 to stage 10 b of the turbine and withdrawn as stream 17. Stream 17 is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. It should be noted that this turbine could produce more than two expanded steam streams, each at different pressure levels. In practice, turbine 10 could, for example, provide power to drive a refrigeration compressor in a commercial olefins unit.

Another portion of stream 15 is directed as stream 18 to the low-pressure steam header, along with stream 19, the expanded high-pressure steam from turbine 9. The majority of the low-pressure steam is typically withdrawn as stream 20 and used for process heating needs in exchanger 21 as shown. The single exchanger 21 in FIG. 1 would typically represents a number of separate exchangers in the commercial unit. The condensate stream 22 from exchanger 21 is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A further portion of the low-pressure steam can be exported as stream 23 to another process or otherwise used within the olefins unit.

FIG. 2 depicts a preferred embodiment of the present invention, wherein reheat of the high-pressure steam and desuperheating of the low-pressure steam is accomplished. Very high-pressure superheated steam from each of the olefins cracking furnaces is combined as shown and directed to the very high-pressure steam header stream 30. It is a characteristic of the current invention that stream 30 is superheated in the furnaces to a significantly higher temperature than the corresponding stream 1 of FIG. 1. Stream 30 is partially de-superheated in the reheat exchanger 31. The resulting very high-pressure steam stream 32 exits exchanger 31 at a temperature roughly similar to that of stream 1 of FIG. 1. The maximum temperature of stream 32 is typically limited by the design and metallurgy of the downstream expansion turbine 33.

Stream 32 is directed to steam turbine 33, which provides similar functionality as turbine 2 of FIG. 1. Turbine 33 is shown as an extracting turbine, with two stages (stage 33 a and stage 33 b) which are typically mechanically coupled. High-pressure steam (typically at about 600 psig) is recovered from stage 33 a as stream 34. A portion of stream 34 is directed as stream 35 to stage 33 b of the turbine and withdrawn as stream 36. Stream 36 is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium.

Another portion of stream 34 is directed as stream 37 to the reheat exchanger 31 where it is reheated against the desuperheating very high-pressure steam stream 30. The reheated high-pressure stream 38 is directed to the high-pressure steam header as shown. It is a characteristic of this invention that the high-pressure steam stream 38 entering the high-pressure steam header of FIG. 2 is at a higher temperature than the corresponding high-pressure steam stream 6 in the conventional steam system of FIG. 1.

Portions of the high-pressure steam from the high-pressure steam header, depicted as streams 39 and 40, can be directed to other steam turbines, depicted as 41 and 42. It is understood that more or fewer turbines can be fed by the high-pressure steam header, depending on the needs of the olefins process. In order to simplify the FIG. 2, only two turbines are depicted.

A further portion of the high-pressure steam can be directed as stream 43 to one or more heat exchangers to provide heating to one or more units in the olefins process. While a single exchanger 44 is shown in FIG. 2, it is understood that it may represent multiple heat exchangers in a commercial olefins facility. The condensate stream 45 from exchanger 44 is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A final portion of the high-pressure steam can be exported as stream 46 to another process or otherwise used within the olefins unit.

Alternatively, the high-pressure steam export can be taken as stream 47 at a point before the high-pressure steam reheater. This could be advantageous if the external high-pressure steam users are not equipped to utilize the hotter high pressure steam represented by stream 46.

In FIG. 2 steam turbine 42 is shown as an extracting turbine, with two stages (stage 42 a and stage 42 b) which are typically mechanically coupled. Superheated low-pressure steam (typically at about 65 psig) is recovered from stage 42 a as stream 48.

A portion of stream 48 is directed as stream 49 to stage 42 b of the turbine and withdrawn as stream 50. Stream 50 is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium. It should be noted that this turbine could produce more than two expanded steam streams, each at different pressure levels. In practice, turbine 42 could, for example, provide power to drive a refrigeration compressor in a commercial olefins unit.

Another portion of stream 48, stream 51, is combined with the expanded superheated steam stream 52 from turbine 41 and the combined stream 53 enters the desuperheater exchanger 54. It is a characteristic of this invention that the low-pressure steam streams 51 and 52 are at a higher temperature than the corresponding low-pressure steam streams 18 and 19 in the conventional steam system of FIG. 1.

Stream 53 is at least partially desuperheated in exchanger 54 to produce the low-pressure steam stream 55. Cooling for the desuperheater exchanger 54 can be supplied by any suitable cooling medium. For example, a relatively cool boiler feed water stream 56 could be used as the cooling medium to produce a relatively warmer boiler feed water stream 57, thereby recovering heat within the steam system and improving the overall efficiency of the process of the present invention.

The low-pressure steam stream 55 from the desuperheater exchanger enters a low-pressure steam header as shown. The majority of the low-pressure steam is typically withdrawn as stream 58 and used for process heating needs in exchanger 59 as shown. The single exchanger 59 in FIG. 2 would typically represents a number of separate exchangers in a commercial unit. The condensate stream 60 from exchanger 59 is withdrawn as shown and at least a portion is typically re-used as boiler feed water for the process. A further portion of the low-pressure steam can be exported as stream 61 to another process or otherwise used within the olefins unit.

FIG. 3 depicts an alternate configuration of the high-pressure steam reheat section of the present invention. It is similar in function to the reheat section of FIG. 2, but the high pressure steam to the second stage of the first turbine is reheated before entering the second stage.

Very high-pressure superheated steam from each of the olefins cracking furnaces is combined as shown and directed to the very high-pressure steam header stream 70. Stream 70 is partially de-superheated in the reheat exchanger 71. The resulting very high-pressure steam stream 72 exits exchanger 71 at a temperature roughly similar to that of stream 1 of FIG. 1 and stream 32 of FIG. 2.

Stream 72 is directed to steam turbine 73. Turbine 73 is shown as an extracting turbine, with two stages (stage 73 a and stage 73 b) which are typically mechanically coupled. High-pressure steam (typically at about 600 psig) is recovered from stage 73 a as stream 74. If desired, a portion of the high pressure steam can be exported from the process a stream 75. The remainder of the steam is directed as stream 76 to the reheat exchanger 71 where it is reheated against the desuperheating very high pressure steam.

A portion of the reheated stream 77 is directed as stream 78 to stage 73 b of the turbine and withdrawn as stream 79. Stream 79 is typically recovered at as low a pressure as feasible (typically under vacuum) and condensed against a near-ambient cooling medium.

Another portion of stream 77 is directed as stream 80 to the high-pressure steam header as shown. For simplicity the remainder of the steam process is not depicted in FIG. 3, but it is understood that it can be similar in nature to that of FIG. 2 (where stream 80 of FIG. 3 corresponds to stream 38 of FIG. 2), or it can be of a different configuration.

FIGS. 2 and 3 depict two configurations which utilize the concept of recovering superheat from a combined vapor stream in order to re-heat at least a portion of a lower-pressure vapor stream which has been extracted from an expansion turbine. Those skilled in the art will recognize that, once the basic concept is grasped, other configurations can be developed, and all such configurations are covered within the scope of this invention.

EXAMPLE OF THE INVENTION

The following Example will serve to illustrate a certain specific embodiment of the herein disclosed invention. This Example should not, however, be construed as limiting the scope of the novel invention as there are many variations which may be made thereon without departing from the spirit of the disclosed invention, as those of skill in the art will recognize.

General

To demonstrate several beneficial aspects of the present invention, both the comparative process depicted in FIG. 1 and the embodiment of FIG. 2 were simulated using commercially available process simulation software.

Comparative Example

Following is an example of a conventional steam system configuration for an olefins manufacturing unit. The design of this conventional steam system is similar to that shown in FIG. 1, and all stream and unit numbers in this example refer to those in FIG. 1. Very high-pressure steam at a temperature of 980° F. and a pressure of 1800 psig is generated from multiple furnaces. High-pressure steam is extracted from turbine 2 at 600 psig, while the low-pressure header operates at 50 psig. No high-pressure steam is exported in this case.

Turbine 2 generates approximately 112,000 HP. Turbine 10 represents the combination of two separate refrigeration turbines which generate a total of 33,800 HP. Turbine 9 represents a number of smaller turbines which combined generate approximately 4,900 HP.

Stream flows and conditions for this example are given in Table 1. Stream numbers correspond to those of FIG. 1. A total of 941,500 lb/hr of very high pressure steam is used.

Example of the Present Invention

Following is an example of a steam system configuration of the present invention for an olefins manufacturing unit. This novel steam system incorporates the high-pressure steam reheat and low-pressure steam desuperheating functions contained within the process of this invention. The steam system of this example is similar to that shown in FIG. 2, and all stream and unit numbers in this example refer to those in FIG. 2. Very high-pressure steam at a temperature of 1090° F. and a pressure of 1800 psig is generated from multiple furnaces. High-pressure steam is extracted from turbine 33 a at 605 psig, and experiences a 5 psi pressure drop across the reheat exchanger so that the high-pressure header operates at 600 psig. The low-pressure header operates at 50 psig.

No export steam was taken through either streams 45 or 47. The amount of steam withdrawn as stream 37 was set so as to maintain a temperature of 980° F. in stream 38. In addition, the amount of low-pressure steam produced by both the current and previous examples was kept constant.

Further, in recognition that the olefins furnaces can provide a finite duty for steam generation, the furnace convective bank duty required to generate the very high pressure steam stream of the present invention (stream 30) was approximately equal to that required for the previous example (stream 1). Stream 30 contains 917,000 lb/hr of steam at 1800 psig and 1090° F.

Note that although the total convective bank furnace duties in these two examples are approximately equal, there are differences in how the duty is utilized to generate steam. In the invention of the present invention, boiler feed water is preheated by superheated expanded steam in exchanger 54 of FIG. 2. Therefore, compared with the comparative example, in the process of the present invention the furnace convection section provides relatively less preheating of the boiler feed water and relatively more superheating of the very high pressure steam. The result is that slightly less very high pressure steam is generated, but it is at a higher final temperature.

Stream flows and conditions for this example are given in Table 2. Stream numbers correspond to those of FIG. 2.

The improved efficiency of the present invention is manifested in increased power production in turbines 33 and 42 as compared to turbines 2 and 10. Table 3 compares the turbine power generation results from the above examples. It is clear that the higher efficiency of the system of the present invention (FIG. 2) produces over 4000 HP more power than a conventional steam system, from similar furnace steam duty.

It should be noted that the improved efficiency of the process of this invention can be manifested in a number of ways. One is the ability to produce more usable power from the same furnace steam duty, as demonstrated above. It may be desirable instead to produce similar power as the conventional system, but with reduced furnace steam duty or through the export of high-pressure steam. These and other methods of taking advantage of the increased efficiency of the present invention will be apparent to those skilled in the art.

An Example has been presented and hypotheses advanced herein in order to better communicate certain facets of the invention. The scope of the invention is determined solely by the scope of the appended claims.

For the purposes of the present invention, “predominantly” is defined as more than about fifty percent. “Substantially” is defined as occurring with sufficient frequency or being present in such proportions as to measurably affect macroscopic properties of an associated compound or system. Where the frequency or proportion for such impact is not clear, substantially is to be regarded as about twenty percent or more. The term “a feedstock consisting essentially of” is defined as at least 95 percent of the feedstock by volume. The term “essentially free of” is defined as absolutely except that small variations which have no more than a negligible effect on macroscopic qualities and final outcome are permitted, typically up to about one percent.

TABLE 1 Comparative Example Steam Flow Temperature Pressure Stream (lb/hr) (Deg F.) (psia) 1 941,475 980 1,815 5 511,524 126 2 6 429,951 710 615 7 97,090 710 615 8 332,861 710 615 11 0 N/A N/A 14 0 N/A N/A 17 158,385 126 2 18 174,476 310 65 19 97,090 392 65 23 0 N/A N/A

TABLE 2 Example of This Invention Steam Flow Temperature Pressure Stream (lb/hr) (Deg F.) (psia) 30 917,000 1,089 1,815 32 917,000 980 1,810 36 505,714 126 2 37 411,286 712 620 38 411,286 980 615 39 76,086 980 615 40 335,200 980 615 43 0 N/A N/A 46 0 N/A N/A 47 0 N/A N/A 50 139,720 126 2 51 195,480 511 65 52 76,086 622 65 55 271,566 308 62

TABLE 3 Turbine Power Comparative Example Example of This Invention (FIG. 1) (FIG. 2) Turbine Number Power (HP) Turbine Number Power (HP) 2a + 2b 110923 33a + 33b 108882 9 4889 41 4888 10a + 10b 33800 42a + 42b 40269 Total 149612 Total 154039 

1. A power recovery process employing at least two classes of expansion turbines, which process comprises: (a) expanding a first stream of superheated vapor at first inlet conditions, including temperature and pressure, to obtain at least one first expanded stream of superheated vapor at first intermediate conditions using at least one primary class expansion turbine to thereby recover a first amount of power; (b) combining two or more vapor streams into a single very high-pressure superheated vapor stream; (c) cooling the resulting single very high-pressure stream from step (b) by indirect heat exchange with at least a portion of the first expanded stream from step (a) to provide all or a portion of the first stream of superheated vapor for expansion in step (a), and a resulting heated first expanded stream at second intermediate conditions; (d) expanding at least a portion of the resulting heated stream from step (c) at second inlet conditions to obtain at least one second expanded stream of superheated vapor at third intermediate conditions using at least one secondary class expansion turbine to thereby recover a second amount of power.
 2. The process of claim 1 wherein three or more vapor streams are combined in step (b) into a single very high-pressure superheated vapor stream.
 3. The process of claim 1 wherein three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are derived from a petrochemical process.
 4. The process of claim 1 wherein the vapor comprises a light organic component containing from about 2 to about 4 carbon atoms.
 5. The process of claim 1 wherein the second intermediate temperature is no more than 100 Fahrenheit degrees below the first inlet conditions temperature.
 6. A power recovery process employing at least two classes of expansion turbines, which process comprises: (a) expanding a first stream of superheated steam at first inlet conditions of temperature and pressure to obtain at least one first expanded stream of superheated steam at first intermediate conditions using at least one primary class expansion turbine to thereby recover a first amount of power; (b) combining three or more streams of very high pressure steam into a single very high-pressure superheated stream; (c) cooling the resulting single very high-pressure stream from step (b) by indirect heat exchange with at least a portion of the first expanded stream from step (a) to provide all or a portion of the first stream of superheated vapor for expansion in step (a), and a resulting heated first expanded stream at second intermediate conditions including a second intermediate temperature; (d) expanding at least a portion of the resulting heated stream from step (c) at second inlet conditions to obtain at least one second expanded stream of superheated steam at third intermediate conditions using at least one secondary class expansion turbine to thereby recover a second amount of power.
 7. The process of claim 6 which further comprises treating at least a portion of one or more second expanded stream of superheated steam from step (d) to thereby provide at least a portion of the resulting single very high-pressure stream of step (b).
 8. The process of claim 7 wherein three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are generated in a process for thermal cracking of suitable petroleum derived feed stocks to produce olefins.
 9. The process of claim 8 wherein the olefins being produced are ethylene and/or propylene.
 10. The process of claim 6 wherein three or more of the vapor streams combined in step (b) into a single very high-pressure superheated vapor stream are generated in a process for the manufacture of light olefins by the pyrolysis of hydrocarbons in a plurality of furnaces from which heat is removed in a plurality of very high pressure streams of superheated steam.
 11. The process of claim 6 wherein the first inlet pressure is at least 900 psig.
 12. The process of claim 6 wherein the first inlet temperature is at least 800° F.
 13. The process of claim 6 which further comprises partially desuperheating the second expanded stream of superheated steam from step (d) by indirect heat exchange with a cooling medium to provide a supply of superheated low-pressure steam.
 14. The process of claim 13 wherein the cooling medium is boiler feed water, and wherein at least a portion of the heated boiler feed water is used to produce at least a portion of the very high pressure steam streams of step (b).
 15. The process of claim 6 wherein the second intermediate temperature is no more than 100 Fahrenheit degrees below the first inlet conditions temperature. 